/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
        if(!t1 && !t2){return nullptr;}
        else if(t1 && !t2){return t1;}
        else if(!t1 && t2){return t2;}
        else{
            t1->val+=t2->val;
            t1->left=mergeTrees(t1->left,t2->left);
            t1->right=mergeTrees(t1->right,t2->right);
        }
        return t1;
    }
};